//实现一物理像素的下边框
one-px($color)
    position relative
    //元素可以单独进行缩放，伪类元素为块元素
    &:after
      position absolute
      display block
      content ""
      width 100%
      height 1px
      background $color
      bottom 0
      @media screen and (-webkit-device-pixel-ratio:2)
        //.nav谁用混合选择器就是谁
        transform scaleY(.5)
      @media screen and (-webkit-device-pixel-ratio:3)
        //.nav谁用混合选择器就是谁
        transform scaleY(.33333333333333333)



bg-image($name)
  @media screen and (-webkit-device-pixel-ratio:2)
        //.nav谁用混合选择器就是谁
    background-image url($name+"@2x.png")
  @media screen and (-webkit-device-pixel-ratio:3)
    background-image url($name+"@3x.png")